1. Field of the Invention
The present invention relates to telecommunications and, more particularly, to methods and systems for controlling the power levels at which mobile devices transmit when communicating with wireless access points.
2. Description of Related Art
Wireless local area networks (WLANs) are increasingly being used for data, voice, and other media. Such WLANs may use protocols, such as the IEEE 802.11x wireless LAN protocols, and may use unlicensed frequency bands, such as frequency bands in the 2.4 GHz range. A WLAN includes a wireless access point that provides a wireless coverage area within which mobile devices can communicate over an air interface. Such mobile devices may include, for example, wireless telephones, wireless PDAs, and/or laptop computers equipped with a WLAN network interface card (NIC).
Because of FCC restrictions in these unlicensed bands, a wireless coverage area in a WLAN typically has only a very limited geographic extent, such as only part of a building. To provide coverage in a wider geographic area, a WLAN can include multiple wireless coverage areas provided by multiple wireless access points. To prevent interference between signals in different wireless coverage areas, different channels may be used in adjacent wireless coverage areas. For example, when using 802.11 with direct sequence spread spectrum (DSSS), eleven channels are allowed in North America. These eleven channels are partially overlapping, but channels 1, 6, and 11 are non-overlapping.
As a mobile device moves between wireless coverage areas, a roaming process may occur in which the wireless access point and/or channel the mobile device uses to communicate changes. In 802.11x systems, roaming is typically initiated by a mobile device when the mobile device loses contact with the wireless access point with which it is associated or when the mobile device judges the communication with that wireless access point to be poor (e.g., a low data rate due to a weak signal). The mobile device then attempts to locate another wireless access point (e.g., using either active scanning or passive scanning) and to associate with it.
Thus, roaming in 802.11x systems is typically a break-before-make process. Moreover, a mobile device typically does not attempt to find the optimal wireless access point (e.g., the closest) as it moves. Instead, a mobile device typically continues to communicate with the wireless access point with which it is currently associated until the wireless connection becomes unacceptable, even though another wireless access point may be closer.
As a result of the limited number of non-overlapping channels and the way roaming typically occurs, designing a WLAN with a large number of wireless coverage areas to cover a contiguous area can be challenging. This difficulty can be exacerbated by the power levels that mobile devices use to transmit. In particular, the power level at which a mobile device transmits is often set by the user and remains fixed as the mobile device engages in wireless communication and moves from one wireless coverage area to another. For example, a laptop computer may have software for controlling a WLAN NIC that includes an option by which the user can set the transmit power level. In many cases, however, the software may use the highest power level (e.g., 100 mW for 802.11) as the default, and many users may never change the default setting. As a result, many mobile devices may communicate with the WLAN at an inefficiently high power level. Such high power levels needlessly reduce the battery life of the mobile devices. High power levels can also cause congestion because a mobile device transmitting at a high power level is more likely to continue communicating with the wireless access point of the previous wireless coverage area when the mobile device moves into a new wireless coverage area. In addition, because mobile devices may transmit at power levels comparable to the power levels at which wireless access points transmit in many WLANs, high power levels transmitted from mobile devices can interfere with the transmissions from wireless access points.
Accordingly, there is a need to provide ways to better control the power levels at which mobile devices transmit when communicating with wireless access points.